1. Field of the Invention
This invention relates generally to an anti-loosening fastener for joining metal panels. Specifically, the invention is directed to a sheet metal screw for attaching two thin panels together, which screw exhibits an increased stripping torque over those of the prior art.
2. Description of the Prior Art
With presently available sheet metal screw, the range between drive torque and strip torque is unacceptably small. For this reason, it is virtually impossible, given dimensional tolerances and variances in material, to find a single torque setting on power screwdrivers that will drive the hardest-to-drive screw without stripping the easiest-to-drive screw. Such a situation results in a relatively high number of screws being stripped during installation, leading to reduced clamping force between two metal panels being joined or secured together, or requires substantial operator time for removal and replacement of stripped screws. A type of screw that probably would provide a solution to this problem by increasing the range between drive torque and strip torque, making it more difficult to strip the screw with the torque required to drive it into the panels being joined together.
One such attempt is disclosed in U.S. Pat. No. 5,183,359, issued to Barth on Feb. 2, 1993. Barth discloses a head 20a and a threaded shank 10a, and the head 20a has a clamping face 210a which include s a number of polyhedral nibs 23a extending axially therefrom and separated from one another by substantially planar uninterrupted portions of the clamping face 210a. The nibs 23a include a leading wall 233a and a trailing wall 235a, facing in the direction of tightening and opposite thereto, respectively. The leading wall 233a may be inclined at a steeper angle with respect to the plane of the clamping face 210a than is the trailing wall 235a, or vice versa. The leading wall 233a and trailing wall 235a meet to form an apex 231a which is also inclined with respect to the plane of the clamping face 210a, so that the nibs 23a increase in axial height as one views outward toward the periphery of the clamping face 210a. And the trailing wall 235a angle may be greater than the leading wall angle 231a may still inhibit further tightening of the fastener member, while the trailing wall 235a may resist backout due to vibration, as shown in FIGS. 1A and 1B.
Unfortunately, the direction of vibration cannot be predetermined. The trailing walls 235a of the nibs 23a are designated for absorbing one-way vibrational torque only. Therefore, this type of fastener cannot be used in a nondirective high frequency vibration environment.